Whatever its effect on the world at large, the Bush administration has been a boon for America's documentary-makers. In recent years, Michael Moore, Errol Morris and Robert Greenwald have all enjoyed brisk business with works that, implicitly or otherwise, address everything from 9/11 to Iraq to the supine US media. Yet arguably the most mature, measured and sensitive of these films comes from Spike Lee, a director not previously known for exhibiting these qualities.

When the Levees Broke - which screened this week on BBC4 - is a forensic four-hour examination of the humanitarian crisis that engulfed New Orleans in the wake of hurricane Katrina. Expertly weaving news footage with eyewitness accounts, it exposes a federal government caught napping - either not knowing or not caring about the plight of its underclass.

This subject matter is meat-and-drink for Lee, who has long prided himself on his role as a political film-maker, a champion of black America and a thorn in the side of the establishment. The difference is in the handling. Until now Spike Lee's reputation has been built on his tub-thumping dramatic features. Films such as Do the Right Thing or Bamboozled combined righteous rage with youthful exuberance, cajoling the viewer to react. Confronted by the horror of Katrina, however, he has opted to let the evidence do the talking. The result is a devastating picture of social division and official failure, and a film that rings out louder than the bombast of old.